1. Field of the Invention
The present invention relates to a substrate heating apparatus which heats a substrate in a vacuum quickly, a heating method, and a semiconductor device manufacturing method employing the heating method.
2. Description of the Related Art
A semiconductor manufacturing technique frequently requires a process for heating and cooling a semiconductor substrate quickly. In particular, activation annealing of a wide bandgap semiconductor represented by silicon carbide (SiC) requires a high temperature of approximately 2,000° C.
An electron impact heating apparatus is conventionally proposed in which thermoelectrons are generated by a single-loop or multiple-coil filament arranged in a vessel placed in a vacuum, and are caused to collide to generate heat.
Usually, in the electron impact heating apparatus, the thermoelectrons are accelerated by applying an acceleration voltage between a filament and a conductive heater on which a substrate as an annealing target is arranged, thus generating a high temperature (Japanese Patent Nos. 2,912,613, 2,912,616, and 2,912,913). FIG. 6 is a perspective view showing the structure of a single-loop filament used in a conventional electron impact apparatus, and FIG. 7 is a perspective view showing the structure of a multiple-coil filament.
In the conventional electron impact apparatus, for example, a graphite vacuum heating vessel is proposed in which a tungsten filament has a single-loop or multiple-coil structure.
A triple-coil filament causes thermoelectrons to actively collide against a side surface of a conductive heater to increase the temperature of this surface. Utilizing heat conduction from the side surface of the conductive heater, a plate body such as a substrate arranged on the conductive heater is heated uniformly.
Thermoelectrons emitted from the filament do not have directivity when emitted from the filament, but are emitted in all directions about the filament as the center.
For this reason, the thermoelectrons are emitted not only in a direction to enter the side surface of the conductive heater which is to be desirably heated actively, but also toward the center of the filament and downward.
The thermoelectrons also emitted toward the center of the filament and downward are converged on the center portion of the conductive heater by a reflection plate provided under the filament, thereby, uniformity of heat is degraded in the conductive heater.
FIG. 8 is a graph showing the result obtained by measuring the temperature distribution of a conductive heater by thermography when the filament is a multiple-coil heater. In FIG. 8, the axis of abscissa represents the distance (mm) from the center of the multiple-coil filament, and the axis of ordinate represents a temperature (° C.) corresponding to the distance.
In this manner, for example, in the conventional triple-coil filament, the thermoelectrons reflected by the reflection plate under the filament are undesirably focused on the center of the upper portion of the conductive heater because of the influence of the electric field. Consequently, the temperature difference at a location away from the center by 50 mm reaches near 100° C.
More specifically, in an apparatus in which electron impact heating is performed by a conventional coil filament, the temperature at the center of the conductive heater is extremely high. Also, heat radiation from the side portion of the heating surface of the conductive heater is large. Accordingly, uniform annealing characteristics within the substrate surface are not obtained.
Devices fabricated from a substrate which is heated in this manner vary largely in characteristics, leading to a low yield.
When a large-diameter substrate is heated, the electron impact amount at the center tends to further increase. This may increase the nonuniformity of the surface temperature distribution.